A wireless transmission apparatus in a wireless communication system includes an amplifier (Power Amplifier; hereinafter, sometimes referred to as “PA”) that amplifies power of a transmission signal. The wireless transmission apparatus typically operates the PA near the saturation region of the PA to enhance the power efficiency of the PA. However, operating the PA near the saturation region increases nonlinear distortion. To suppress the nonlinear distortion to reduce adjacent channel leakage power (ACLR: Adjacent Channel Leakage Ratio), the wireless transmission apparatus includes a distortion compensation device for compensating the nonlinear distortion.
One of the distortion compensation systems used for the distortion compensation device is a “pre-distortion system.” Hereinafter, “pre-distortion” may be referred to as “PD.” The distortion compensation device of the PD system previously multiplies a transmission baseband signal yet to be inputted to the PA by a distortion compensation factor having an inverse characteristic to the nonlinear distortion of the PA. The distortion compensation device of the PD system thereby improves the linearity of the output of the PA to suppress the distortion of the output of the PA. The transmission baseband signal multiplied by the distortion compensation factor may be referred to as a “pre-distortion signal (PD signal).” The resulting PD signal is a signal previously distorted according to the inverse characteristic of the nonlinear distortion of the PA before input to the PA.
For example, some distortion compensation devices of the PD system have a table containing a plurality of distortion compensation factors, and read a distortion compensation factor according to the power of the transmission baseband signal from the table. The distortion compensation factors stored in the table are successively updated so that an error between the transmission baseband signal serving as a reference signal and the signal that is outputted and fed back from the PA (hereinafter, sometimes referred to as a “feedback signal”) is minimized.
For example, some conventional power amplification systems including a distortion compensation device of the PD system limit the frequency band of the feedback signal when updating the distortion compensation factors. In such power amplification systems, the pass bandwidth of the feedback signal is changed by a band-limiting filter. If the passband has a predetermined narrowness, predetermined low-order distortions are compensated. If the passband has a predetermined width, predetermined high-order distortions are compensated.
Related-art examples are described, for example, in Japanese Laid-open Patent Publication No. 2012-090158, in Japanese Laid-open Patent Publication No. 2012-060254, in Japanese Laid-open Patent Publication No. 2003-008360, in Japanese Laid-open Patent Publication No. 2002-359583, in Japanese Laid-open Patent Publication No. 2007-020157, and in International Publication Pamphlet No. WO 2009/131076.
FIG. 1 is a diagram for explaining a problem. Suppose that the transmission baseband signal is a multicarrier signal including a plurality of signals having respective different carrier frequencies. If the multicarrier signal is subjected to a PA operating in a nonlinear region, inter modulation distortions (hereinafter, sometimes referred to as “IMs”) can occur. That is, if the transmission baseband signal is a multicarrier signal, the feedback signal can include IMs. For example, as illustrated in FIG. 1, if a multicarrier signal including a signal having a carrier frequency f1 and a signal having a carrier frequency f2 is amplified in a nonlinear region, the feedback signal may include IM3s, IM5s, and IM7s which are the third-, fifth-, and seventh-order IMs, respectively. As illustrated in FIG. 1, the IMs occur at the skirts on both sides of the carrier frequencies f1 and f2 and in locations at certain distances from the carrier frequencies f1 and f2 on the frequency axis. Suppose that the distortion compensation device can compensate up to IM7s. In other words, the frequency band capable of distortion compensation (hereinafter, sometimes referred to as a “distortion compensation band”) covers up to IM7s.
The IM3s, IM5s, and IM7s occurring in locations at certain distances from the carrier frequencies f1 and f2 occur in a symmetrical manner on the frequency axis with respect to a center frequency f0 of the multicarrier signal. The center frequency f0 of the multicarrier signal including the signal having the carrier frequency f1 and the signal having the carrier frequency f2 is given by “f0=(f1+f2)/2.”
Conventionally, the frequency band limitation on the feedback signal is performed by using a frequency range of ±fc symmetrical on the frequency axis with respect to a predetermined reference frequency fs as the passband. More specifically, the filter used for the frequency band limitation has cutoff frequencies of “fs+fc” and “fs−fc.” FIG. 1 illustrates the frequency characteristic of the filter. The reference frequency fs is typically 0 Hz.
The center frequency f0 of the multicarrier signal usually coincides with the reference frequency fs. However, in specific cases, as illustrated in FIG. 1, the center frequency f0 of the multicarrier signal can deviate from the reference frequency fs. If the center frequency f0 of the multicarrier signal deviates from the reference frequency fs, the center frequency f0 of the feedback signal also deviates from the reference frequency fs. If the center frequency f0 of the multicarrier signal deviates from the reference frequency fs, the IMs become asymmetrical on the frequency axis with respect to the reference frequency fs.
Examples of the specific cases include when a communication system is being switched from an old one to a new one and the center frequency f0 of the old system which has been coincident with the reference frequency fs is temporarily changed. When switching from an old system to a new system, the old and new systems can be temporarily made to coexist and placed in operation instead of completely stopping the old one before starting the operation of the new one. In such a case, for example, the center frequency f0 of the new system is shifted by −Δf from the center frequency f0 of the old system.
In another example of the specific cases, respective different carrier patterns may be assigned to a first area and a second area. In such a case, for example, the center frequency f0 in the second area is shifted by −Δf from the center frequency f0 in the first area. As a result, the center frequency f0 which coincides with the reference frequency fs in the first area is −Δf different from the reference frequency fs in the second area.
If the IMs are asymmetrical on the frequency axis with respect to the reference frequency fs, the IMs lying in the passband below the reference frequency fs can be different from those in the passband at and above the reference frequency f. As a result, IMs can be extracted asymmetrically on the frequency axis with respect to the reference frequency fs. In the example illustrated in FIG. 1, the passband below the reference frequency fs includes only the IM3 among the IMs occurring in locations at certain distances from the carrier frequencies f1 and f2. The passband at and above the reference frequency fs includes the IM3 and IM5. In other words, among the IMs occurring in locations at certain distances from the carrier frequencies f1 and f2, the IM5 and IM7 below the reference frequency fs are cut off by the frequency band limitation. In contrast, only the IM7 is cut off at or above the reference frequency fs. That is, suppose that the conventional frequency band limitation is performed on the feedback signal of which the center frequency f0 deviates from the reference frequency fs. In such a case, the feedback signal after the frequency band limitation includes IMs asymmetrical between the range below the center frequency f0 and the range at and above the center frequency f0. In other words, among the IMs included in the feedback signal, the frequency band-limited IMs are asymmetrical on the frequency axis. If the distortion compensation of the PD system is performed by using such a feedback signal including asymmetrical IMs, the accuracy of the distortion compensation decreases due to the asymmetry of the IMs.
The foregoing conventional power amplification systems have not taken into account the problem of a decrease in the distortion compensation accuracy due to the deviation of the center frequency f0 of the multicarrier signal from the reference frequency fs.